Water-in-oil emulsions having a relatively high ratio of water phase to oil phase are known in the art as high internal phase emulsions ("HIPEs" or "HIPE" emulsions). Continuous processes for preparing HIPE emulsions are disclosed, for example, in Lissant; U.S. Pat. No. 3,565,817; Issued Feb. 23, 1971 and Bradley et al; British Patent Application 2194166A; Published Mar. 2, 1988.
HIPE emulsions which contain polymerizable monomers in their external oil phase have been made and polymerized in order to study the geometric configuration of the oil and water phases of such emulsions. For example, Lissant and Mahan, "A Study of Medium and High Internal Phase Ratio Water/Polymer Emulsions," Journal of Colloid and Interface Science, Vol. 42, No. 1, January, 1973, pp. 201-208 discloses the preparation of water-in-oil emulsions which contain 90% internal water phase and which utilize styrene monomer in the oil phase. Such emulsions are prepared by subjecting the combined oil and water phases to agitation using an emulsifier and are subsequently polymerized to form a rigid porous structure having a cellular configuration determined by the phase relationship of its emulsion precursor.
Preparation of HIPE emulsions suitable for polymerization to porous structures, e.g., foams, useful for carrying and/or absorbing liquids are also known. For example, Barby et al, U.S. Pat. No. 4,797,310, Issued Jan. 10, 1989; Jones et al, U.S. Pat. No. 4,612,334; Issued Sep. 16, 1986; Haq et al, U.S. Pat. No. 4,606,958, Issued Apr. 19, 1986; and Barby et al, U.S. Pat. No. 4,522,953, Issued Jun. 11, 1989 all disclose porous polymeric materials which can be prepared from HIPE emulsions and which are useful for delivering liquids such as cleaning solutions to hard surfaces via products such as wipers and cleaning cloths.
The prior art has also recognized that the nature and characteristics of the porous polymeric foam materials formed by polymerizing HIPE emulsions is very much dependent on both the type of components which makeup the polymerizable HIPE emulsion and the process conditions used to form the emulsion. For example, Unilever, European Patent Application No. 60138, Published Sep. 15, 1982 discloses a process for preparing absorbent porous polymers (i.e., foams) from high internal phase emulsions comprising at least 90% by weight of water with the oil phase containing polymerizable monomers, surfactant and a polymerization catalyst. Edwards et al, U.S. Pat. No. 4,788,225, Issued Nov. 29, 1988 discloses the preparation of porous polymer materials which are rendered elastic by selecting certain monomer types (styrene, alkyl(meth)acrylates, crosslinker) and by using certain processing conditions to control the cell size of the eventually resulting porous polymer. Unilever, European Patent Application EP-A-299,762, Published Jan. 18, 1989 discloses that the use of an electrolyte in the water phase of polymerizable HIPE emulsions can affect the size of the openings between cells of the eventually resulting porous polymeric foam material.
Notwithstanding the fact that the existence and synthesis of polymerizable HIPE emulsions is known in the art, preparation of HIPE emulsions suitable for polymerization to useful absorbent foam material is not without its difficulties. Such HIPE emulsions, and especially HIPE emulsions having a very high ratio of water phase to oil phase, tend to be unstable. Very slight variations in monomer/crosslinker content in the emulsion, emulsifier selection, emulsion component concentrations, and temperature and/or agitation conditions can cause such emulsions to "break" or to separate to at least some degree into their distinct water and oil phases. Even if stable emulsions can be realized, alterations in emulsion composition and processing conditions can significantly affect the properties and characteristics of the eventually realized polymeric foam materials, thereby rendering such foam materials either more or less useful for their intended purpose. Such HIPE emulsion preparation difficulties can become even more troublesome when there is a need to produce polymerizable emulsions via a continuous process on an industrial or pilot plant scale in order to provide commercially useful or developmental quantities of polymeric absorbent foam materials.
Given the foregoing considerations, it is an object of the present invention to provide a process for preparing certain types of high internal phase emulsions that can be polymerized to form foam materials especially useful as an absorbent for aqueous body fluids, i.e., foams which are useful in absorbent products such as diapers.
It is a further object of the present invention to provide such a HIPE emulsion preparation process which can be carried out on a continuous basis.
It is a further object of the present invention to provide such a continuous HIPE emulsion preparation process which can be operated on a commercially meaningful scale.